This invention relates to a method for measuring the fitness for use of a storage battery in the case of electric loading of the storage battery.
It is particularly important for users of batteries to know the state of charge and high-current loadability of the batteries. For example, the state of charge and the aging state, or the drop in capacity beginning to be shown by the battery, are decisive for the ability of a starter battery to start a motor vehicle having an internal combustion engine, since the current strength which can be drawn from the starter battery, or the power output thereof, is limited by such ability. It is particularly important to determine the state of charge or the starting ability of a battery in cases in which, for example, intermittent engine operation occurs. This is because, in times of engine standstill, the vehicle electrical network, with its consumers of electricity, continues to operate even though the generator is not generating current. Monitoring the charge and starting ability of the battery must ensure in such cases that the energy content of the battery remains sufficient to start the engine.
The most varied methods are known for measuring the state of charge of storage batteries. In many cases, integrated measuring instruments are used (Ah meters), account being taken of the charging current, if appropriate with evaluation using a fixed charging factor. Since the useful capacity of a battery depends strongly on the magnitude of the discharging current and the temperature, these methods are also incapable of providing satisfactory statements on the useful capacity which can still be extracted from the battery.
For example, it is known from DE-C 2242510 to use a factor which depends on the temperature and the state of charge of the battery itself to evaluate the charging current with the use of a method for measuring the state of charge.
DE-A 4007883 discloses a method in which the starting ability of a storage battery is determined by a measurement of storage battery voltage and battery temperature and comparison with a family of state of charge curves valid for the type of battery to be checked.
DE-A 19543874 discloses a method of calculation for the discharging characteristic and measurement of the residual capacity of a battery in the case of which current, voltage and temperature are likewise measured, the discharging characteristic being approximated by a mathematical function with a curved surface.
DE-C 3901680 describes a method for monitoring the cold starting ability of a starter battery, wherein the starter battery is temporarily loaded with a resistor, the voltage dropping across the resistor is measured, and it is established therefrom by comparison with empirical values whether the cold starting ability of the battery is still sufficient. The starting operation serves in this case to load the starter battery.
Finally, DE-A 4339568 discloses a method for determining the state of charge of a motor vehicle starter battery wherein the battery current and open-circuit voltage are measured and used to infer the state of charge, the battery temperature also being taken into account in addition. In this case, the charging currents measured during various time intervals are compared and a residual capacity is determined therefrom.
However, determining the state of charge does not always suffice, in particular, since it is to be taken into account, for example, that in the case of a greatly aged battery the voltage will collapse upon loading despite an adequate state of charge and, thus, the usefulness of the battery is not ensured.
Accordingly, it would be advantageous to be able to determine the characteristic of the battery which goes beyond establishing the battery charge and from which it is possible to infer the battery""s fitness for use.
The invention relates to a method for measuring fitness for use of a storage battery subject to electric loading comprising: determining a load profile (current profile I(t) or power profile P(t)) as a function of time t for the storage battery; recording an actual voltage response U(t) of the storage battery to the load profile or calculating a voltage response U(t) of the storage battery to the load profile; and determining a fitness for use value SOH for the storage battery based on the difference between a lowest (highest) voltage value Umin (Umax) during application of the load profile to the storage battery, and based on a voltage limiting value U1, wherein U1 is a voltage value which may not be undershot (overshot) by the voltage U(t) at any time t during which the load profile is applied to the storage battery.